The invention relates to a surface acoustic wave transducer with a piezoelectric substrate and at least two electrodes on the surface of the substrate wherein the electrodes comprise fingers which overlap with the fingers of another electrode in a projection parallel to the surface of the substrate and a filter consisting of SAW transducers.
Surface acoustic wave (SAW) transducers can be used to design electronic filters which are optimized for a broad, flat passband with steep transition edges to the stopbands. A transducer comprises at least two electrodes which are located on the surface of a piezoelectric substrate, e. g. a crystal of quartz or lithium tantalate in a specific cut and orientation. Typically, the electrodes are manufactured by surface metallization of the substrate and consist each of a contact pad with fingers which project from one edge of the contact pad. The contact pads of two electrodes are arranged parallel to each other with the fingers extending inwardly between them and being interdigitated. Therefore, the fingers of both electrodes overlap in projection parallel to the contact pads.
When an electric signal is applied to the electrodes, the voltage causes elastic deformations of the substrate in the gaps between the fingers. The deformations propagate as surface acoustic waves in direction parallel to the contact pads and can be received by another transducer on the same substrate. The frequency of the maximum response of the SAWs to an electric signal is related to the fundamental period .lambda. of the fingers, i. e. an optimum coupling is achieved if the wavelength .lambda. of the surface acoustic waves is approximately equal to the width of two fingers on adjacent electrodes with their corresponding gaps. The location and overlap of the fingers of the transducer correspond approximately to the Fourier transform of its frequency response which consists of a main lobe and several side lobes for a transducer with a single passband as it is well known in the art.
It is of high importance that transducers have the lowest possible insertion loss and a flat passband without curvature at the edges. Besides of bidirectional loss and SAW propagation loss, the insertion loss of such a transducer with a suitably tuned feeding circuit is dominated by the Q-value if the ohmic losses are sufficiently low. The Q-value of the transducer is defined by EQU Q=.vertline.m(Y).vertline./Re(Y)
wherein Y is the transducer admittance and Re and Im denote the real and imaginary value respectively. A low Q-value can easily be obtained with .lambda./4 wide fingers separated by gaps of equal width. This structure has a high coupling to the surface acoustic waves and a correspondingly low Q-value. Even if a filter with a broad passband is to be designed which requires a small number of fingers for a given length in the main lobe of the transducer, a low Q-value may be attained in this way.
Due to the surface inhomogeneities caused by the fingers, a part of the propagating surface acoustic waves is reflected at each finger. This effect distorts the surface acoustic waves, especially if large numbers of fingers are present on the substrate. The effect is most pronounced for .lambda./4 wide fingers with gaps of equal width because in this case all SAW reflections add constructively. Consequently, fingers with a width of .lambda./4 are avoided in SAW devices in the state of the art. As alternatives, split fingers consisting of two adjacent fingers with a width of .lambda./8 on the same electrode or combinations of fingers with a width of 3.lambda./8 and .lambda./8 have been proposed. Both structures are low reflecting because the partial waves reflected at different fingers do not interfere in phase. However, the Q-value is higher than that of a structure with .lambda./4 wide fingers.